An FM-CW (Frequency Modulated Continuous Wave) radar is mounted on, for example, an automobile, so as to be applied to such functions as collision prevention and inter-vehicle distance control.
FIG. 1 is a diagram illustrating a basic configuration of the FM-CW radar. A function generator FG in an oscillator unit generates a triangular-wave-shaped modulated signal (control signal) of, for example, 500 Hz, or of that order, so as to perform frequency modulation on a VCO with the above triangular wave. The frequency modulated signal output from the VCO is radiated from an antenna as a transmission signal. A signal reflected after being hit on a target is received by the antenna, and a beat signal is obtained by that a mixer mixes the reception signal with the transmission signal. By analyzing the beat signal, it is possible to obtain distance and relative speed to the target.
An important matter of the FM-CW radar is that a maximum and a minimum frequency shifts thereof should not be changed from the criterion of the center frequency of the frequency modulated signal, and that the frequency should be changed with time in a line shape (linearly). However, in general, a VCO frequency modulation characteristic is not linear.
FIGS. 2A-2C are diagrams illustrating a VCO frequency modulation characteristic. FIG. 2A is a diagram illustrating the VCO frequency modulation characteristic, in which an output frequency is not changed linearly relative to a control voltage. Therefore, as illustrated in FIG. 2B, in order to output an accurate triangular-wave-shaped frequency modulated signal having an oscillation frequency changed linearly, it is necessary to correct a gradient (change rate) of the triangular-wave-shaped control voltage, as illustrated in FIG. 2C. Specifically, in case of producing oscillation of a triangular wave that is linearly changed with center frequency f0, lower limit frequency f1 and upper limit frequency f2, the VCO frequency modulation characteristic can be corrected by changing, for example, the gradient between a control voltage V0 corresponding to the center frequency f0 and a control voltage V1 corresponding to the lower limit frequency f1 (sections A, D) and the gradient between the center frequency f0 and the upper limit frequency f2 (sections B, C), as illustrated in FIG. 2C.
As a linearity calibration unit for linearly changing the output frequency, conventionally, it is known to generate and store a correction data relative to the VCO frequency modulation characteristic at the time of factory shipment, and to correct the control voltage using the correction data. Also, it is a known method to detect the VCO frequency modulation characteristic by using a received signal from a target (Patent document 1).
Further, in addition to having no linear characteristic, the VCO frequency modulation characteristic is changed due to temperature change and secular change.
FIGS. 3A-3C are diagrams illustrating the change of a VCO frequency modulation characteristic. As illustrated in FIG. 3A, when the VCO frequency modulation characteristic is changed due to temperature change and secular change, in order to output a frequency modulated signal [FIG. 3B] having frequency characteristics (center frequency and frequency shift) which are identical before and after the change, it is necessary to correct the control voltage according to the above change, as illustrated in FIG. 3C.
In order to cope with temperature change, it is necessary to generate in advance the correction data of the control voltage on a temperature-by-temperature basis, which requires a vast amount of data, and a temperature sensor as well. Moreover, correction accuracy stays low. Further, in regard to secular change, it is not possible to cope with it by a method of storing the correction data, and accordingly, it is not possible to detect the change of the frequency modulation characteristic in real time.    [Patent document 1] Japanese Laid-open Patent Publication No. 2003-28951.